Radio Servicing Information

For use by legally qualified and licensed repairers only

Countries have laws regulating who is legally allowed to repair or work on electrical equipment that will be plugged into the mains electricity supply. This is to protect the public from the risk of electric shock. If you are not lawfully allowed to repair electrical equipment yourself, you must find an electrician, a licensed electrical mechanic or electrical contractor or electronic technician to do any work on the equipment.

Different countries use electricity supply at different voltages. The United States uses 110-120 volts AC. The United Kingdom and Australia (amongst many other countries) uses 240 volts AC. Any electric shock at Voltage 110v or above is likely to be lethal.

Many types of early consumer electronic equipment was constructed at a time when electrical safety was not as carefully considered as is required under manufacturing laws today, and many early radio sets etc have mains electricity wiring and metal mains voltage terminals fully exposed to be touched once the metal chassis is removed from the wooden case or bakelite housing.

This is why any electrical work to restore or repair an old radio must be done by a licensed qualified technician.

POWERING UP A RADIOThere are different methods for powering up a radio.If the radio had been dormant for many years, it may not have been working when it was put in storage, so it may have a 50 year old fault, as opposed to a fault caused by age.However if the radio was working and has just stopped, then normal fault finding can be attempted.

PROTECTION DEVICEA simple protection device can be made. It is simply a 100 watt incandescent light globe and a switch. It can be placed in series with the mains cord during testing. This will act like a fuse. If there is a short circuit, the bulb will shine at full brilliance. If the radio is working normally, the bulb will glow just a little.

FOR THE FIRST TIME (SLOWLY).A variable transformer called a VARIAC is a useful device for slowly bringing a radio to life. You can slowly turn the knob and apply the mains voltage, from zero to 240 volts over several hours. The range from 180 to 240 is crucial, as the valve rectifier suddenly starts to work and the HT is applied. Be careful and take your time. In later model radios, if the rectifier uses semiconductor diodes, then this doesn't apply, as the HT voltage is always a proportion of the applied AC.

FOR THE FIRST TIME (STEP BY STEP)Another method, is to test the radio section by section, and turn the radio OFF between each test.Start by drawing a diagram of what valves go where. Then CAREFULLY remove the valves and place them in a box with tissue paper. Grip them by the base, not by the glass, as this may break the bond between the base and glass. Pull them straight up, do not wiggle. If the valves have no makings, attach a paper label to their base, with the type or location. Don't assume the valves are correct, or that they are working, or in the proper socket, as someone may have attempted to fix the radio before you.

Check the mains wiring, cord and plug.Put a meter across the primary of the transformer, set to 250 volts A.C. or greater.Turn the radio OFF.Turn the power socket switch OFF.Plug in the mains plug.Turn the power socket switch ON.Check for smoke. Listen for sparking.Turn the radio ONCheck for smoke. Listen for sparking.Check there is 240 volts AC reading on the meter.

Turn the radio OFF and the power socket switch OFF.Move the meter, to read the heater voltage (a convenient place is across the audio output valve socket).Set the meter to 10 volts AC or greater for 6 volt valves. Set it appropriately for different valves.Turn the power socket switch ON and the radio ON .A reading of the correct heater voltage, means that the power transformer is working, and that there are no short circuits on the heater wiring.

Repeat this procedure for the high tension winding.This is usually 250 AC (or 385 AC) and can be checked at the rectifier valve socket.

If this is all correct, then plug in the audio output valve ONLY.Its heater should glow correctly.

If this is all correct, then plug in all the other valves, but not the rectifier.They should light up correctly with a dull red glow. Now for the tricky part.Plug in the rectifier, attach the meter to the high tension point, with the meter is set for 500 volts DC.Turn ON the radio, and watch the meter needle climb to 250 volts (or above).If this doesn't happen within 20 seconds, turn the radio off quickly.Start fault finding in the power supply.Immediately check the filter capacitors for heat (they should be cold, not hissing, nor sparking, nor warm).Turn the radio OFF quickly if any noise (apart from the speaker) is heard.

If this is all working, then start fault finding in the other stages.Continue checking the filter capacitors and power transformer for heat, during the fault finding.

If the radio trips a circuit breaker, then use the series light globe to check for a full short circuit. If it is not a full short circuit then it is leakage. The circuit breaker can be a Core Balance or Earth Leakage type. The breaker trips becuase the AC going into the radiothrough the active wire, SHOULD EXACTLY balance, the AC coming out on the neutral wire. If any current goes through the earth wire, the system in unbalanced and it trips the breaker. This is to protect against short circuits to earth. However, if there is ANY leakage to earth, it will trip. Often there is a capacitor from active to chassis, to reduce noise. This becomes old and may conduct a small current, so disconnect it, or replace it. Look for damage to the transfomrer, or arcing on the mains switch or terminal strip. Look for wiring errors, like active/neutral/earth reversal. Look for short circuits. Look for any dirt or frayed wires, look inside the power plug for gunk. Clean anything off and let it dry for a day.As an aside, this is why the house lighting circuit is completly different from the power circuit. When a fuse blows or a breaker trips at night time, you are not left in the dark.